How does the installer decide

[Grizzly]

04 May 2021  at 2:49, Bret Busby wrote:
Re: How does the installer decide (at least in part)

>On 04/05/2021, Grizzly via ubuntu-users <ubuntu-users at lists.ubuntu.com> wrote:
>> Hi All
>>
>> Tried to install 21.04 on a PC with 20.04.3 & Win7 it only offerd to erase
>> (20.04) and install or "something else"

<snipped>

>If you have, or, can create, by reducing the size of an existing partition or
>partitions, free space, or, a free partition, 

There is plenty of space on either disk (each 500Gb) one has only Win7 the 
other 20.04.3 Focal, I'm (pretty) sure "if" I removed the 20.04 disk (just for 
the install) it would share the Win7 disk, but I wanted to just "do the 
install" as I have with other distros (one of the Puppy's, Bodhi or even 
another Ubuntu flavour) that would install beside "out-of-the-box"

[the question was more WHY than how to work around]

>I recommend choosing the "something else" option, 

I have in the past used gParted to re-size partitions, it always took an age to 
do, but when installing it seems to do the sizeing and moveing about much 
faster?

>then you can choose to boot an existing installation, or the 21.04
>installation. 

>an unusable (I forgot the password, because it was too difficult to use) MS
>Windows 8 installation. 

There are a nmber of (linux based) Windows PW recovery apps that I have used 
many times (but not so far tested on a dual boot) if it's worth it for Win8 ???

>On each system, at each reboot, I can choose which OS to boot.

>So, I recommend "something else".

Ok, given 21.04 will be replaced in six months, so wont need to have a huge 
amount of space, what partition sizes do you recommend (I assume I can share 
the swap?) it would be nice to share /Home but I can see that may be a problem 
if apps have different versions for 20.04/21.04, so maybe each should be self 
contained?

I have only once done anything close to this and that was for a friends 
AppleAir (I think) with pure EFI boot and he needed an external disk that would 
be self containd and not effect the base machine in any way, but that was all 
done from the terminal (I still have the instructions somewhere)

>If you have a legacy BIOS only system, apparently, as I found, I could
>ignore the threat of catastrophic destruction, that appears, to
>successfully complete and use the 21.04 installation.

Some (if not all) are BIOS only boxes

Many thanks for any help